1. Field of the Invention
The invention relates to a method of polishing copper surfaces, and more particularly to a method of polishing surfaces of copper wiring in an ultra large scale integrated circuit.
2. Descriptions of Related Art
Copper has low resistivity, superb anti-electromigration, and low thermal sensitivity, and can produce a shorter resistance and capacitance delay (RC delay) and improve the reliability of circuits. Thus, copper wire is an ideal substitute for aluminum wire to work as interconnection lines.
As the size of elements of integrated circuits becomes smaller, and the number of metal layers and the diameter of the silicon substrate increase, the degree of the flattening of each layer has become one of the important factors that affect the linewidth of the etching of the integrated circuits, and been a bottle-neck in the development of microelectronics. Chemico-mechanical polishing (CMP) is so far the most advanced technology for providing a whole flattening for a plurality of layers of copper wirings, dielectrics, and barriers. A model of polishing rate of the conventional CMP is MRP=Kpv, from which it is known that the polishing rate is proportional to the press (p) and the rotational speed (v). However, the conventional CMP method has large press, and after polishing, scratches, edge collapses, and roughness easily occur on the surface, all of which result in low yields. In the meanwhile, with the development of the integrated circuit technology, new materials with small permittivity are necessary to work as a dielectric layer to shorten the RC delay, and to reduce the power loss. However, such materials are always porous materials, the larger the porosity is, the smaller the permittivity is, and the worse the compression resistance is, thereby resulting in cracks when bearing high mechanical strengths. Porous materials with permittivity of 2.5 are widely used in 45 nm technology node, in which the magnitude of the mechanical strength directly affects the performance of the CMP method. To lower the down force, electric and chemico-mechanical polishing (ECMP) method has been proposed, which, however, has such disadvantages as unrepeatable production, poor stability, and high production cost and so on. If the conventional CMP method is applicable to smaller technology nodes, customers prefer CMP method to ECMP method, as far as the cost and risk are concerned.
Therefore, to achieve an effective polishing rate under low pressure, and at the same time to solve problems such as scratches, edge collapses, and damages on material have been critical in the development of the CMP technology.